The present invention relates to a color image forming apparatus wherein a toner image is formed on an image forming member in an electrophotographic method and the toner image is then transferred onto a transfer material so that an image may be obtained.
As an image forming method for obtaining a color image through an electrophotographic method, there have been known methods wherein latent images corresponding in quantity to the number of separated colors of an image on a document are formed and developed repeatedly on an image forming member to be super imposed thereon, and then the superimposed color toner images are transferred so that a color image may be obtained, as disclosed in Japanese Patent Publication Open to Public Inspection Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985 (hereinafter referred to as Japanese Patent O.P.I. Publication), for example.
The first example of a color image forming apparatus employing an image forming method wherein latent images corresponding in quantity to the number of separated colors of an image on a document read by a color scanner are formed and developed repeatedly on an image forming member is represented by one wherein exposure units and developing units each corresponding in quantity to the number of separated colors (for example, 3 colors of yellow, magenta and cyan or 4 colors including also black) are provided around a belt-shaped image forming member. A second example thereof is represented by one wherein exposure units and developing units each corresponding in quantity to the number of colors are provided around a drum-shaped image forming member. A color image forming apparatus equipped with the belt-shaped image forming member will be explained as follows.
In the color image forming apparatus, a belt-shaped image forming member comprising a flexible belt on which photoconductive substances are coated or evaporated is spread over tension rollers so that the flexible belt may rotate while staying in contact with a guide member representing a reference position, with the help of tension caused by pressure-contact with the tension rollers. Thus, the surface of the belt-shaped image forming member is moved in the direction of the surface while the surface is kept constantly at a certain position in the direction perpendicular to that surface. Owing to the manner mentioned above, image forming means including a charging unit, an exposure unit and a plurality of developing units each containing different color toner (yellow, magenta, cyan and black) provided around the belt-shaped image forming member, can keep a fixed distance from the belt-shaped image forming member when it rotates.
With regard to the speed of a photoreceptor in a color image-forming apparatus, the speed of a motor for driving a photoreceptor is controlled, by means of a servo technology known widely, to be at a predetermined constant speed.
However, even when a photoreceptor-driving motor is controlled to be at a predetermined constant speed, the speed of the surface of the photoreceptor itself is not necessarily at the speed set in advance. For example, when a drum is used for a photoreceptor, the speed of the surface of the photoreceptor varies when a diameter of the drum changes, despite the constant speed of the photoreceptor-driving motor. Further, when a photoreceptor belt is used, the moving speed of the photoreceptor varies depending on the variation in diameters of belt-driving rollers and in belt thickness.
That is, when a photoreceptor drum, a photoreceptor belt or a belt-driving roller is replaced in the case of maintenance or the like, the speed of the surface of the photoreceptor varies because of variation in terms of dimension of replaced items.
In systems having photoreceptor drum or in multi-rotation systems, there is no such problem. When a photoreceptor belt is used, however, the variation in belt thickness and in diameters of belt-driving rollers cause a problem in that superimposed images slip in the secondary scanning direction.
Namely, a plurality of image writing units are positioned at predetermined intervals and they are controlled to start writing images successively with a delay of a predetermined period of time as an image position moves from the upstream side on a photoreceptor belt so that images may be superimposed. When a moving speed of the photoreceptor belt changes, moving time of an image position is changed. Therefore, even when image writing units are controlled to start writing images successively with a delay of a predetermined period of time, a position to start writing images on the photoreceptor belt sometimes has passed the exposure point, or has not reached the exposure point.
Further, on an ordinary image forming apparatus, the speed of rotation of a motor for driving a conveyance system is controlled so that the speed of a transfer sheet may be the same as that of a photoreceptor belt to avoid image disturbance. When a moving speed of the photoreceptor belt varies, however, speed difference still takes place and images are disturbed (i.e., so-called "transfer slip" takes place).
In a conventional method for solving the problem mentioned above, a photoreceptor belt is provided on its circumference with a large number of slits which are read by a photocoupler or the like, so that the speed of the photoreceptor belt itself may be controlled to be constant by signals read by the photocoupler or the like. In this case, however, an exclusive photocoupler is needed, resulting in disadvantages in the aspects of space and cost.
From the viewpoint of an image writing unit, on the other hand, those positioned at predetermined intervals tend to be displaced in terms of intervals when temperature changes. This phenomenon is usually caused by thermal expansion of a plate on which image writing units are positioned. For example, when an interval for positioning of writing units is 15 mm, it corresponds to a distance of 450 mm between the first color and the fourth color which results in the interval change of about 0.1 mm with temperature change of 20.degree. C.
Thus, an image of each color is deviated by the temperature change of 20.degree. C., when the writing of each image is only controlled to start with a delay of predetermined period of time.
On a color image forming apparatus wherein a plurality of toner images (Y, M, C, K) are superimposed on an image forming member for forming a color toner image, when an exposure-start position on the belt-shaped photoreceptor mentioned above cannot be set with a deviation of less than or equal one pixel unit, about 80 .mu.m for example, from a plurality of exposure units, image quality of a color toner image deteriorates has been a problem. Especially, when a belt-shaped image forming member is used, control of image registration has been difficult, compared to that of a drum-shaped image forming member.
To prevent color slip in the secondary scanning direction in a color image forming apparatus, there is considered a method wherein each latent image can be formed from the same point on an image forming member by establishing a registration mark on the image forming member, detecting the registration mark with a photo-coupler and by causing a plurality of exposure units to start exposure in succession based on the detected signals.
In the color image forming apparatus mentioned above, however, a photo-coupler and an exposure unit need to be positioned extremely precisely and exposure units also need to be positioned at accurate intervals (accuracy of about .+-.0.01-0.1 mm) although a registration mark established on a belt-shaped image forming member is read by a single photo-coupler or by a plurality of photo-couplers and timing for start exposure of each exposure unit is determined accordingly. Actually, it has been difficult to position a plurality of sensors and exposure units with positional intervals which are extremely accurate mechanically (the highest accuracy possible as of the date of filing of the subject application was about .+-.0.3-0.5 mm). In addition, when exposure units are removed on the occasion of maintenance or the like, it has been very difficult to position them again at their original positions.
Further, in the method mentioned above wherein a pattern image (a registration mark) provided in advance on an image forming member (a photoreceptor) or a transfer member is detected in terms of position by a photo-coupler, or in a method wherein toner images are formed on a photoreceptor or on a transfer member and they are detected by a photo-coupler that is used exclusively for positional detection, an exclusive photo-coupler needs to be provided for controlling, which has resulted in disadvantages in space and cost and in remarkable deterioration of image quality.
In addition to the above, in the secondary scanning direction, namely in the rotating direction of an image forming member, various factors such as speed variation of the driving source, eccentricity in intermediate power transmission means, and surface speed variation caused by expansion and contraction of a belt-shaped image forming member due to temperature change may cause positional variation of each image writing in the secondary scanning direction, deterioration of accuracy of registration in the secondary scanning direction, inability of accurate superposition of each color image and deterioration of image quality.
In a color image forming apparatus, positional slip (registration slip) of superposition of toner images to be superimposed results in a change in shade which causes deterioration of image quality.
Actually, accuracy of superposition is required to be equal to or higher than the accuracy on the order of one pixel unit, 80 .mu.m-100 .mu.m for example.
The registration slip includes the slip in the primary scanning direction and that in the secondary scanning direction. Though various causes are considered for the slips in both primary and secondary scanning directions, the slip in the primary direction will be explained as follows. Two causes for the slip are discussed below.
The first cause is that there is a precision limit in the positional relation between a writing unit and an image forming member when the positional relation relies on only accuracy of assembly. Especially when superposition of two or more images each having different color formed by a plurality of writing units is performed during one rotation of the image forming member, each writing unit itself is deviated in position although an image position is registered in each writing unit. Thereby, the image position for each color is deviated.
The second cause is a problem on the side of an image forming member that an image position varies continually because of movement or stagger in the thrust direction of the image forming member taking place during running of image formation because of temperature change. Even when adjustment is made so that an image for each color may be written at the same position in the primary scanning direction, a position of the image made previously is changed for each writing, making it impossible for images to be superimposed accurately.
A method used to solve the registration slip due to the first cause is discussed below called an index sensor, is provided in the vicinity of an image forming member on a writing position, the photoreceptive sensor is irradiated by a laser beam for writing use during the primary scanning for writing, and signals obtained therefrom are utilized as a reference signal for start writing images.
In the method mentioned above, one writing unit is used and when an image on one sheet is obtained through several rotations of a photoreceptor, the positional relation between a photoreceptive sensor and an image is the same for each color and it causes no problem.
When a plurality of writing units are used and two or more images each having different color are superimposed through a single rotation of an image forming member, however, a plurality of index sensors each corresponding to each writing unit are needed. Positioned relation between each index sensor and each image forming member may be kept to a certain level with mechanical accuracy, but it is limited at most to 0.3-0.5 mm. Therefore, an image position for the first color formed on the image forming member cannot be identified accurately, which makes it impossible to superimpose accurately an image for the second color on the image for the first color.
With regard to the second cause, when a belt-shaped image forming member is used as an image forming member, a belt is always accompanied by stagger, and in this case, a positional relation between an index sensor and an image forming member at a writing position changes continually by an amount of the stagger. Though an amount of stagger can be reduced by mechanical accuracy, or an error can be minimized by incorporating some stagger control system, the limit for reducing an amount of stagger is about 0.2 mm.
To solve the foregoing problems a registration mark is put on an image forming member in advance, the registration mark is detected by an exclusive photo-coupler provided in the vicinity of a writing position, a stagger amount of the image forming member is calculated from the detection signals, and the start of writing images is delayed by the amount of stagger for avoiding the registration slip.
When one writing unit is used and one sheet of image is obtained through several rotations of a photoreceptor as described above, the same photo-coupler is used for each color (since only one photo-coupler is used) and not problem is caused.
When a plurality of writing units are used and two or more images each having different color are superimposed through a single rotation of an image forming member, however, a plurality of photo-couplers each corresponding to each writing unit are needed, and a positional error is caused in positioning the photo-couplers. The error in this case can also be reduced to a certain extent through mechanical accuracy, but the limit for reducing the error is about 0.1-0.2 mm at the best. In this case, an exclusive sensor used for controlling has caused disadvantages in space and cost aspects, and yet satisfactory positioning is not achieved and image quality has deteriorated extremely.
Further, with regard to a scanning exposure apparatus, an optical system disclosed in Japanese Patent O.P.I. Publication No.95361/1983 is known for its compactness. In this case, both sides of the primary scanning line need a pattern image on a photoreceptor or a transfer member because a laser beam in which the scanning direction is opposite to the writing direction is used.